Re: That encoder stuff.

Thank You Elliot for stepping in and giving a great explanation.

Arne. Are you reading?

Bertho Boman
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> "Elliot Burke" <elliot@...> wrote:
>
> The Talbot effect is easy to understand if you consider the many diffraction
> orders of a square wave grating. Considering only a point source, each
> diffraction order will propogate an image of a grating in the direction u,
> when a is the grating period, w is the wavelength, and m the order of
> diffraction:
>
> a sin u = m w
>
> (I wish there were theta's and lambda's on this keyboard)
>
> At the Talbot distance these orders coincide in phase, reconstructing or
> imaging the grating.
> This is a remarkable effect, and i recommend trying to view it.
>
> Another interesting effect is caused by partial spatial coherence. A source
> of light is said to be spatially coherent if it can be focused to a point.
> A laser is a good example. Light focused through a small pinhole can be
> made nearly spatially coherent.
> This is important because the projection of an object grating onto the
> analyzing grating changes in phase with different points on the source
> perpendicular to the grating lines. Thus (in general) a broad source cannot
> be used. If the source has a grating of the correct frequency directly on
> it, then the phases of all the projected grating lines will be the same.
> This can give a tremendous increase in brightness over a single slit, and
> allows the use of very high frequency gratings.
>
> These grating systems at high frequencies are best analyzed as
> interferometers, as indicated above. This will clear up lots of the
> confusion, if you feel like wading through an optics textbook. Another step
> forward in understanding them requires a more rigorous diffraction theory,
> which is a bit beyond the charter of this list.
>
> Elliot Burke